erofs: clean up cached I/O strategies

After commit 4c7e4255 ("erofs: remove useless cache strategy of
DELAYEDALLOC"), only one cached I/O allocation strategy is supported:

  When cached I/O is preferred, page allocation is applied without
  direct reclaim.  If allocation fails, fall back to inplace I/O.

Let's get rid of z_erofs_cache_alloctype.  No logical changes.

Reviewed-by: Yue Hu <huyue2@coolpad.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Yue Hu <huyue2@coolpad.com>
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Link: https://lore.kernel.org/r/20221206060352.152830-1-xiang@kernel.org